This invention relates generally to the optical art and, more particularly, to a method and apparatus for producing a phase conjugate replica of a light wave.
For any light wave propagating through a medium there can exist, in principle, a time-reversed replication of that wave. Thus, an appropriately patterned but irregular wave front can travel through a light-scattering medium and emerge as a uniform wave front, provided it is a replica, reversed in time, of a wave that was originally deformed by the same medium. Such a time reversed wave front is generally referred to as a phase conjugate replica of the original wave.
It is well known that phase conjugate replicas of light waves can be generated by nonlinear effects in an irradiated medium. For example, nearly time-reversed waves have been obtained by stimulated Brillouin scattering of laser beams of high optical intensity. Phase conjugation has also been obtained using the photorefractive effect in selected materials. In the photorefractive effect, it is commonly believed that charges (of unknown origin) migrate in the presence of light from regions of high optical intensity to regions of low optical intensity, eventually reaching a static charge distribution. This charge distribution creates a strong static electric field which alters the index of refraction of the material by the first order electro-optic (Pockels) effect. When two light beams, (1) and (2), interact within a nonlinear material, a periodic modulation of the refractive index of the material is obtained. A "reading" beam directed oppositely to the beam (2) can be scattered from the refractive index pattern, also called a "grating", to create an output beam opposite in direction to the beam (1). Under appropriate conditions, the output beam is the phase conjugate replica of beam (1). This process is known as "four-wave mixing" and has been performed with such nonlinear materials as sodium vapor and liquid carbon disulfide. The process is described in U.S. Pat. No. 4,145,671 to Hellwarth.
Phase conjugation by four-wave mixing has been performed using single-domain crystalline barium titanate (BaTiO.sub.3) as the nonlinear mixing material, as described in the following publications: Feinberg et al., "Photorefractive Effects and Light-Induced Charge Migration in Barium Titanate", J. Appl. Phys. 51(3), March 1980; Feinberg et al., "Phase-Conjugating Mirror With Continuous-Wave Gain", Optics Letters, Vol. 5, No. 12, December 1980; and Lam et al. "Photorefractive-Index Gratings Formed by Nanosecond Optical Pulses in BaTiO.sub.3 " Optics Letters, Vol. 6, No. 10, October 1981. However, these procedures require external pumping of the BaTiO.sub.3 by counterpropagating laser beams in the same manner as earlier four-wave conjugators.
BaTiO.sub.3 has been used in a self-pumped phase conjugator, as described in White et al., "Coherent Oscillation by Self-Induced Gratings in the Photorefractive Crystal BaTiO.sub.3 ", Appl. Phys. Lett. 40(6), Mar. 15, 1982. In the White device, part of the incoming beam is deflected for oscillation between a pair of opposed mirrors. The oscillating beams correspond roughly to the external pumping beams of four-wave mixing, producing a phase conjugate beam which has approximately 10 percent of the intensity of the incoming beam. Although this arrangement does not require external pumping, the two external mirrors make it relatively bulky and restrict the geometric configurations in which it can be used. Alignment of the mirrors is also critical and the light path outside the mixing material involves substantial reflection and absorption losses. A further disadvantage of a phase conjugator using the principles disclosed by White is a relatively long optical path. The physics of the interaction require that the oscillating light beam be reasonably coherent with the input beam. The long light path of White thus effectively limits the arrangement to use only with light having a relatively long coherence length, such as substantially monochromatic laser light. If light having a shorter coherence length were used, it probably would not interact in the desired manner.
Therefore, it is desirable in many applications to provide a compact and versatile apparatus for phase conjugation.